This invention relates to production well completion and, more particularly, to a tool for aligning casing in relation to a wellhead in an oil or gas drilled well bore.
After a well bore is drilled, it is conventional to position a casing in the well bore to protect the wall of the drilled well bore and ensure that production pipes can be safely positioned in the well bore. The casing is lowered into the well on a set of slips that grip and hold it in position in relation to the wellhead. In order for the slips to be properly positioned around the casing, it is necessary to center the casing inside the wellhead so that the center of the well bore and the center of the casing are concentrically aligned. Casing hanger slips are used to hold up the casing and cement the casing inside the well bore.
A casing section is a hollow tubular body having 40 or more feet in length and from 4 to 20 inches in diameter. The casing sections are threaded together to extend into the well bore and prepare the drill hole for production of oil, gas, water, or other natural resources. Once the casing hanger slips are set and the excess casing is removed, if necessary, the casing adapter spool is added on top of the wellhead and bolted down tight. The structure forms the bottom of a Christmas tree, through which the natural resources are delivered to the surface. Drilling then continues until another hanger spacing is run to either complete the well or to isolate a zone that might cause problems in the drilling process. In all cases, the casing must be lined inside the well bore in order to connect it with another casing section so that drilling can continue and the well can be completed.
The task of casing alignment is even more complicated in an offshore location, where a large gap of approximately 90 feet exists between the wellhead on the ocean floor and the bottom of the offshore platform. The gap tends to cause bending of the casing pipe, which further complicates the problem. Rig personnel use chains and air tuggers to try to align the casing with the wellhead. This practice presents additional difficulties since the rig has only a limited number of fixed points, such as the legs of the derrick on which a cable can be secured to pull the casing into alignment. The personnel have to jury rig, at great danger to personnel on the rig, to try to align the casing and the wellhead to set the slips and proceed with the drilling operations. Valuable rig time is lost and the cost of the completion operation rises.
An additional problem is that the fixed points available on the rig are seldom located at points where the casing can be engaged and pulled into alignment. Still another obstacle is the limited distance that the casing can be moved in the rotary table.
The present invention contemplates elimination of drawbacks associated with conventional methods and provision of a casing alignment tool that can be set up and moved to operation in a relatively short time, thereby saving the expense and improving safety conditions on the drilling rig.